Why this matters more here than most places
The difference between the two heater types is where the scale ends up.
In a tank, calcium settles to the bottom and sits there. It insulates the burner and costs you efficiency, but the tank keeps working for years while it happens. It is a slow, forgiving failure.
A tankless unit has no reservoir. Water moves through a narrow copper or stainless heat exchanger and is heated on the way past, so every mineral that comes out of solution deposits on the exchanger walls — the surface doing the actual work. There is nowhere else for it to go.
Pair that with some of the hardest water in Texas and you have the reason the maintenance interval here is genuinely shorter than the one in the manual.
What scale does, in the order you notice it
Three things happen as the layer thickens, and they show up in this sequence.
- Heat transfer drops. The unit has to push the same BTUs through a mineral layer, so it fires longer and harder for the same result. This costs you money for months before it costs you comfort.
- Flow drops. The internal passages are narrow to begin with. As they constrict, the unit's usable gallons per minute falls — which is why the first real symptom is usually lukewarm water when two fixtures run at once.
- Error codes start. Most units flag a fault once scale passes a detectable threshold. By this point it has been building for a long time.
Worth knowing
The efficiency loss arrives long before the symptom does. If you have noticed the temperature wavering at high demand, the unit has been running inefficiently for months already — so the flush is overdue rather than early.
What a flush actually involves
It is a descaling circuit, not a rinse. A pump moves a food-grade descaling solution through the heat exchanger in the reverse direction of normal flow for around 45 minutes to an hour, dissolving the mineral layer and carrying it out.
The service isolation valves get used for this — the pair of valves with hose ports that should have been fitted either side of the unit at installation. If your heater does not have them, that is the first thing to sort out, because without them a flush means breaking into the pipework every time.
The inlet screen filter comes out and gets cleaned at the same time; it catches sediment and is frequently the reason a unit is short of flow. The burner and venting get a look while the panel is off, and the unit is fired and checked for correct temperature rise before we leave.
How often, honestly
Manufacturers usually say annually, and that figure assumes average water. Ours is not average.
On unsoftened Boerne water, once a year is the minimum and twice is defensible for a hard-working unit in a full house. On a well at 25-35 grains, twice a year is the sensible default.
If the house has a correctly sized softener, the interval genuinely stretches — every 18 months to two years is reasonable, because you have removed most of what forms the scale in the first place. That is worth weighing when you price a softener: it is partly a heat exchanger insurance policy.
What does not change is that the flush still matters. Softened is not the same as mineral-free.
Signs yours is overdue
Any of these means it has been too long.
- Hot water that goes lukewarm when a second fixture opens.
- The temperature swinging during a single shower rather than holding steady.
- Noticeably longer to get hot at the tap than it used to be.
- Any scale or maintenance error code on the display.
- A rumbling or crackling from the unit while it fires.
- More than a year since the last one — or no record of one ever being done.